Closed circuit method of circulating a substantially solid free drilling fluid

ABSTRACT

The basic fluid is water, oil or an oil-water mixture. The primary weighting materials are dissolved salts, e.g., sodium chloride, calcium chloride or mixed brines, and in these the calcium or sodium ions inhibit hydration and swelling of drilled solids. When non-cationic fluids are used, an inhibitor is added to prevent such hydration and disintegration--an electrolyte or a cationic surfactant. Any viscosifier added is one having neither a structured viscosity nor gel strength, hydroxyethyl cellulose, for example. When the drilling fluid is pumped to the surface, most of the drilled solids are in their original condition and size and are readily removed and discarded by screening and other available physical means. There is none of the build-up of solids associated with older systems and, hence, no need for discarding, thinning, and make-up.

This application .Iadd.is a reissue of U.S. Pat. No. 3,844,361 which.Iaddend.is a division of Ser. No. 101,177 filed Dec. 23, 1970, nowabandoned.

The present invention lies in the field of drilling fluids or muds usedin forming wellbores in the earth for the production of oil, gas, waterand other minerals. More specifically, the invention applies to a methodof formulating such a drilling fluid so that it can be continuouslycirculated down through the hollow drill string to pick up the cuttingsformed by the drill bit, carry them to the surface in the annulussurrounding the bit, and pass them through a series of mechanicaldevices and settling tanks to remove the drilled solids and return thedrilling fluid to the drill string for recirculation--with essentiallyno loss of either vehicle or additives. Even more specifically, theinvention discloses drilling muds eliminating or substantially reducingmany of the adjustment steps necessary with prior art muds--discarding aportion of an overladen mud, diluting the retained fraction, and makingup to original composition by adding various constituents.

In the prior art of formulating drilling fluids, whether the fluid is tobe circulated as in rotary drilling or placed in the bottom of the borehole as in cable tool drilling, a clay such as bentonite is usuallyadded to water to prepare an aqueous mud. In other cases drilled shalesare allowed to accumulate in the mud as drilling progresses in order tobuild up such properties as weight, viscosity and gel strength.Sometimes muds with oil as the continuous phase are used to prevent holeproblems, and in other cases flocculants are added to aqueous mud to aidin dropping out solids at the surface.

The result of using such a clay-based mud system has been a build-up ofundesirable solids, either in the circulating mud or in the form ofexcess mud which is stored in reserve or disposal pits. This build-up,if not properly dealt with, will cause increases in unit weight(specific gravity), viscosity and gel strength to such high levels thata number of undesirable events may occur.

The mud cake on the wellbore wall may become so thick that swabbingoccurs in pulling the bit, causing sloughing or caving of the wall andfurther increases in viscosity and gel strength. Gas bubbles from adrilled formation may become trapped in the mud. Drilling rate maydecrease because of the thick filter cake on bottom. The fluid may evenbecome so thick as to be unpumpable.

To avoid such dire results, mud engineers keep a close watch on thecirculating mud and take many preventive steps, all rather expensive.Flocculants are added in the settling of pits in attempts to bring aboutagglomeration and settling of the hydrated and dispersed drilled solids,a step likely to remove some of the originally added bentonite as well.A fraction of the mud is discarded or laid aside, and the balance isthinned to the desired unit weight with water. Of course, in so thinningthe concentrations of some if not all constituents of the mud will bereduced below the desired levels. The only recourse available to the mudengineer is to add more of the very materials he has just thrown out inthe discarded fraction--primarily bentonite, but also many of the otheradditives he may be using for fluid loss control and various otherproperties.

It is the primary object of the present invention to reduce for theoperator the cost of the drilling fluid he circulates in his well, andto do this by constantly recirculating the same fluid, discardingvirtually nothing but the drilled solids. (Mud must be added withincreasing depth, of course, and various changes in the composition ofthe mud must be made for changes encountered in the formation beingdrilled, e.g., permeability and formation fluids, but these must be madein any event.)

A second object is to furnish such a drilling fluid having neither gelstrength nor structured viscosity, so that drilled solids are readilyseparable from the drilling fluid at the surface by one or bothscreening and settling from quiescent drilling fluid.

A third object is to furnish such a drilling fluid which is inhibitedagainst hydration and disintegration of formation solids, either becausean oil is used as part of the mud or because an inhibitor is added to awater phase.

A fourth object is to furnish a drilling fluid which enables the drillerto obtain higher drilling rates than with clay muds.

A fifth object is to supply a drilling fluid having no tendency to builda thick filter cake on the borewall. Expressed positively, this objectis to furnish a drilling fluid which forms a coating, rather than acake, on the borewall.

A sixth object is to provide a drilling fluid having a filtrateinhibited against hydration and swelling of drilled clays, to avoid holetrouble which may be caused by sloughing clays or heaving shales.

A seventh object is a drilling fluid which reduces the re-startup loadon mud pumps after a period of time in which a quiescent column ofdrilling fluid has been sitting in a wellbore.

An eighth object is a drilling fluid which will permit ready escape ofany gas intruding into the fluid during its passage through thewellbore.

The above and other objects are accomplished according to the presentinvention by the use of a closed circuit mud system wherein the basefluid is an inhibited brine which can be reused indefinitely bydisposing of the drilled solids in essentially a dry state. In order toeffectively maintain a closed circuit mud system, several conditionsmust be satisfied, and these may be summarized as follows:

1. The mud must be conditioned so that the drilled solids do not hydrateand then swell or disintegrate into the mud system. Conditioning may beaccomplished in various ways, such as adding electrolytes to water. Theaddition of cationic surfactants to water is also effective as they willbe substantive to the cuttings and coat them with a water repellinglayer of chemical. Emulsions of water in oil are also effective as thecuttings will not hydrate in oil.

2. The mud must have neither thixotropic properties nor a structuredtype viscosity, as separation of the cuttings at the surface would begreatly hindered. Thixotropy is also termed gel strength, and is areversible gel wherein structure builds up when the mud is quiescent andthen breaks back down when the gelled fluid is agitated. A structuredtype viscosity is one wherein viscosity is primarily obtained byphysical rather than physico-chemical means. Asbestos and attapulgiteare examples of these type materials, each being structured as needlesor filaments lacking the chemically active surface groups thatcharacterize, for example, filaments of hydroxyethyl cellulose (HEC).Their viscosities are characterized by high flat gel strengths, i.e.,there is little change in viscosity when agitated and initial gelstrength is high and constant over a period of time.

3. The mud must supply the properties needed for drilling. Some of theseproperties are carrying capacity, so that cuttings can be removed fromthe borehole, fluid loss control so that the borehole itself will bestabilized while drilling, and sufficient density to keep formationfluids out of the well during drilling.

As an example of a mud satisfying these conditions, a trouble-free wellwas recently drilled in England using one of the low-cost mud systems ofthe present invention. This was Walton No. 1 Well, in Yorkshire, whereHome Oil Company Ltd. of Calgary, Alberta, Canada was the operator. Thismud system was prepared with calcium chloride brine as the base fluid.The cuttings-carrying ability of this brine was increased by adding the"Bex" viscosifier of Chemical Additives Company at the rate of 11/2pounds per barrel of mud, this material having as its active ingredienthydroxyethyl cellulose (HEC) in the ratio of about 50-75 percent byweight. HEC can be used by itself, but is more readily and completelyutilized when added with the activator used in the "Bex" viscosifier,e.g., finely divided magnesia. For details see the disclosure of thecopending application Ser. No. 101,123 of the same inventor, filed Dec.23, 1970, now abandoned, which is hereby incorporated herein byreference.

To control fluid loss, a calcium lignosulfonate was added at the rate ofabout 5 pounds per barrel. This was in the form of "Brigeheal" fluidloss additive, again a product of Chemical Additives Company. Itcontains some finely divided magnesia and some inert calcium carbonateparticles and is not reactive with those constituents of the presentinvention which limit the viscosity of the mud to a no gel strength andno structured viscosity system and inhibit the drilling fluid againsthydration and swelling of drilled solids. It may be replaced by otherfluid loss additives, when any such additive is necessary, which arenon-reactive and do not so affect viscosity and hydration. For detailssee the aforementioned copending patent application.

The major portion of the cuttings from this well were removed by shakerscreens. All cuttings and drilled solids were hauled away from the wellsite in an essentially dry state, i.e., they had not hydrated. Only asmall portion of the drilled cuttings from this operation becameentrained in the mud, and said entrainment was attributed to someunavoidable abrasion by the pump and bit. A desander and centrifuge wereused to remove most of these entrained solids, and the balance, alongwith the concomitant calcium chloride, were allowed to rest in asettling pit. The solids settled out and the supernatant clear calciumchloride brine was pumped back into the circulating mud system. Atypical set of mud properties, as freshly mixed, was as follows:

    ______________________________________                                        Weight     10.8 pounds per gallon                                             Viscosity  36 seconds by Marsh Funnel Viscosimeter                                       6 cp plastic viscosity                                                        3 cp yield point                                                   Gel Strength                                                                             zero or 0/0 gels                                                   ______________________________________                                    

To have properties of inhibition, a base fluid should be

a. Primarily oil, as in a water-in-oil emulsion, or

b. Aqueous with one of the following:

600 parts per million (ppm) calcium or other divalent cation

200 ppm aluminum or other trivalent cation, e.g., chromium

1,500 ppm potassium chloride

5,000 ppm sodium chloride

200 ppm of a cationic surfactant such as coco amine or a quatenaryammonium compound.

Various combinations of these salts and cations may also be used withgood results.

The above example is intended to be only illustrative, of course.Viscosity without structure or thixotropy may be obtained easily throughthe use of various gums or polymers known to furnish aqueous fluids theproperty of pseudoplasticity, or through the use of emulsions of eitherwater-in-oil or oil-in-water or through the use of solutions such assugar in water. It will be apparent, of course, that the calcium brinemay be an aqueous solution of any of a number of other cationic saltsand mixtures of such salts; sodium chloride may be used when a lowerunit weight is acceptable, and zinc chloride may be used to make thebrine when a very heavy mud is required. The salt not only furnishes theweighting material (which does not tend to precipitate when the mud isquiescent), but also typically furnishes the cations for inhibiting themud against hydration of solid material.

What is claimed is:
 1. A closed circuit method of circulating asubstantially solid free drilling fluid comprising (1) formulating adrilling fluid based on water, including an additive to inhibit drilledsolids from disintegrating, water soluble weighting agents and aviscosifier consisting of material having no gel strength and nostructured viscosity, (2) pumping said drilling fluid down the wellborethrough the drilling string and bit and back up to the surface in theannulus surrounding the bit and drill string, (3) screening the drillingfluid to separate out and discard the coarse particles (4) removing themajor portion of sand, silt and other particles from said drilling fluidby .[.settling and decantation.]. .Iadd.mechanical means, .Iaddend.(5)removing solids remaining in said drilling fluid by .[.mechanicalmeans.]. .Iadd.settling and decantation .Iaddend.and, (6) returning thecleaned-up substantially solid free drilling fluid to the wellbore. 2.The closed circuit method according to claim 1 wherein said drillingfluid comprises a brine.
 3. The closed circuit method according to claim2 wherein said brine is a calcium chloride brine.
 4. The closed circuitmethod according to claim 3 wherein a clear supernatant calcium chloridebrine is returned to the wellbore in step
 6. 5. The closed circuitmethod according to claim 3 wherein gum or polymer viscosifiers areemployed.
 6. The closed circuit method according to claim 1 wherein onemajor portion of any cuttings are removed by shaker screens.
 7. Theclosed circuit method according to claim 6 wherein sand, silt and otherparticles are removed by desander and centrifuge.
 8. The closed circuitmethod according to claim 1 wherein a clear drilling fluid is returnedto the wellbore in step
 6. 9. A closed circuit method of circulating asubstantially solid free drilling fluid comprising (1) formulating adrilling fluid based on water and containing therein an additive toinhibit drilled solids from disintegrating, water soluble inorganic saltweighting agents and viscosifier having no gel strength and nostructured viscosity and comprising hydroxyethyl cellulose, (2) pumpingsaid drilling fluid down the wellbore through the drilling string andbit and back up to the surface in the annulus surrounding the bit anddrill string, (3) screening the drilling fluid to separate out anddiscard the coarse particles, (4 ) removing the major portion of sand,silt and other particles from said drilling fluid by .[.settling anddecantation.]. .Iadd.mechanical means, .Iaddend.(5) removing solidsremaining in said drilling fluid by .[.mechanical means.]..Iadd.settling and decantation .Iaddend.and, (6) returning thecleaned-up substantially solid free drilling fluid to the wellbore.